I think it will depend on what you have available to start with. If you have a base of anything, the trip to 650 HP will cost about the same money. Being able to actually use the 650 HP may change everything, and how you intend on using the 650 HP also has to be accounted for. How long you expect the motor to live will have a dramatic effect on the cost, regardless of what you start with.

If your starting with no motor, but have a drag car chassis available, you use the motor/trans that fits the easiest in the car. It would be much more easy to get 650 hp hooked up going down a quarter mile strip then it would be to get 650 hp hooked on on the street while expecting to drive across the country. 650 hp looks a lot different if you put that into a car on a NASCAR track, or a mud bog truck.

Otherwise, have fun. Gene